Files for which no blobs have to be restored, still have to be truncated
to the correct size. Take a file with content "foobar" that should be
replaced by restore with content "foo". The first three bytes are
already uptodate, such that no data has to be written. As file
truncation normally happens when writing data, a special case is
necessary.
This no blobs written special case is unified with the empty file
special case.
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